1. Field of the Invention
The present invention relates to such cameras in which an M.P.U. (Micro Processor Unit) is used.
2. Description of the Prior Art
As the electric power source of an automatic exposure control camera (AE camera) there is generally used a battery in the order of 3 V. At present, a micro-processor unit (hereinafter referred to as MPU by its abbreviation) is sometimes used in such camera to control various photographing operations of the camera such as an exposure controlling operation. For an MPU used in a camera, it is very difficult to reduce its operating voltage to a level lower than 2.5 V because of the technical limitation on the manufacture of an IC (integrated circuit). The minimum value of operating voltage of an MPU now available is usually about 2.5 V. In case of the above mentioned cameras such as AE cameras, a large amount of electric power is consumed during the supply of current to the shutter controlling magnet. Therefore, if the battery is in the state of increased internal resistance (at the time of low temperature or at the time close to the end of the battery life), then the battery voltage drops sometimes under 2.5 V during the supply of current to the shutter controlling magnet. In contrast, a small amount of electric current is consumed in the camera during a photometering arithmetic operation and therefore during this time there never occurs a drop of the battery voltage under 2.5 V. For this reason, cameras using an MPU have been designed in such manner that the arithmetic operations necessary for exposure control are all finished during the time of photometering and the operation of the MPU is stopped after releasing the shutter of the camera. After releasing the shutter, the MPU in the camera is kept in a state in which the MPU can not operate and only the internal resulting from arithmetic operations can remain stored therein. In this state, the MPU can keep the internal data in memory even when the source voltage drops to about 1.5 V and therefore the exposure control can be performed without any problem. However, in order to restore the MPU to its normal state for operation, it is required to externally reset the MPU. Timing of this external resetting is of critical importance. If MPU is reset under the condition in which the source voltage is still below 2.5 V, then the MPU can not be returned to its normal state for operation. In this case, there may occur unexpected troubles such as runaway of the program. Hitherto, this matter has not been fully considered in designing the above mentioned type of camera. None of the known apparatus includes effective means to solve the problem. As a result, known cameras of the above type sometimes have malfunctions.